Fiber optic systems in existence today and those foreseen for the future require means of splitting optical power from one optical fiber into two or more optical fibers. Similarly, it can also be required to recombine optical power present in two or more optical fibers into a single optical fiber. The fused coupler has become the preferred method of achieving these aims.
A fused coupler is a device made by placing two or more fibers in mutual proximity so that the cladding glass layer of the fibers come into contact. The contacting region is usually of the order of one or two centimeters long. Heat is applied to a limited portion of the contacting region whereupon the cladding glass of the fibers fuses together. During this operation the heated fibers may be axially drawn in a manner to elongate the fused region, which simultaneously reduces the cross-sectional area of the fused region, resulting in what has become known as a fused biconical taper. In what is referred to as a conventional coupler, the fibers thus processed have identical physical parameters and dimensions. In what is referred to as a wideband coupler, the fibers to be fused may or may not have identical physical parameters and dimensions.
Of interest for many applications is the ability to manufacture a coupler in which the ratio of power output from the cross coupled fiber to the total power output from both output fibers, referred to herein as the coupling ratio, remains essentially constant as the wavelength of the input optical power is varied over relatively broad extremes. In one case it is desirable to operate within an optical bandwidth near one of the preferred optical wavelength regimes of commercially available laser diodes. In this case the wavelength dependence of conventional couplers is strong enough that wavelength variations from laser to laser can cause substantial variation in system performance. For example, a typical laser diode may be specified to operate at 1300 nm+/.+-.40 nanometers (nm). The coupling ratio of conventional couplers usually varies about 0.1 to 0.2 percent per nanometer. Over the specified wavelength tolerance of such a laser diode the coupling ratio may vary from 42 to 58 percent or more, an undesirably large variation.
In another case such as wavelength division multiplexed systems, it may be desired to couple equally at two wavelengths which are relatively far apart, e.g., 1300 nm and 1550 nm. In this latter case, dual window couplers have been fabricated which possess equal splitting ratios at the two design wavelengths. These devices tend to have rather strong wavelength dependence, however, so that optical sources must be chosen to match the coupler windows or vice versa; otherwise the planned coupling ratios may not be realized.
In both cases the best solution is a coupler with coupling ratio essentially independent of the optical wavelength.
It has been shown in theory and in practice that the wavelength dependence of the coupling ratio in such a fiber optic structure can be reduced to some extent in several ways, but with certain drawbacks. In one case wavelength dependence has been reduced by keeping the coupling length as short as possible while also using small diameter etched fibers which are fused into a nearly circular cross-section. Couplers of this type may be generally classified as bi-axially symmetric couplers in as much as the coupling region is essentially symmetric in the longitudinal direction about the point of minimum taper diameter, and they are also symmetric in cross-section. The techniques employed to make such couplers, etching and/or careful control of the dimensions of the tapered coupling region, can result in couplers which are satisfactorily independent of wavelength, but such processes are difficult to tune and control in a manufacturing environment, leading to unwanted non-uniformity among the couplers thus produced, and low manufacturing yield.
Another prior approach is mentioned in D. B. Mortimore, "Wavelength Flattened Fused Couplers", Electronics Letters, Vol. 21, 742, 1985 and International Publication No. WO 87/00934. Wavelength independence is achieved by pretapering one of the two fibers used to make a coupler. Pretapering, i.e., heating and drawing one of the fibers to a reduced diameter prior to fusion of the two fibers, results in alteration of the propagation constant of the pretapered fiber. The pretapered fiber is subsequently fused to the other fiber. This may result in a tapered structure which is longitudinally symmetric about the point of minimum pretaper diameter, but the cross-section is regarded as asymmetric. (See also T. A. Birks and C. D. Hussey, "Control of power-splitting ratio in asymmetric fused-tapered single-mode fiber couplers", Optics Letters, Vol. 13, No. 8, August 1988).
The resulting coupler is thus made from fibers with different local propagation constants and by virtue of that difference between the fibers in the locale of the coupling region, the bandwidth of the coupler can be increased. The difference in propagation constants between the pretapered and and non-pretapered fibers in such a longitudinally symmetric coupler does not appear to vary substantially across the longitudinal extent of the coupling region. The amount of coupled power, hence the maximum coupling ratio at any wavelength, is seen to depend upon the waist diameter of the pretaper. Considerable control of the flattened bandwidth regime and the coupling ratio is possible with this method. On the other hand, these characteristics depend upon careful control of the pretaper diameter and length as well as the fusing taper shape and length and thus may be subject to non-uniform results in a manufacturing operation. In practice, to make such a coupler it appears that one must first pretaper one fiber to a definite minimum diameter, that diameter being known to yield a specific maximum coupling ratio. Couplers of different coupling ratio, therefore, are seen to require a different pretaper setting.
In another prior approach ["Wavelength independent coupler and method of fabrication thereof", European Patent Application No. 88401245.1, Kevin L. Sweeny et al, Amphenol Corporation.] fibers of different core index of refraction hence different numerical aperture and different propagation constant are fused in the conventional manner. Here the difference in propagation constant is a result of the difference in the doping level of the cores of the two optical fibers. These types of couplers are structurally bi-axially symmetric, but their optical properties are only longitudinally symmetric. This method may not require modification of the fibers prior to fusion. The disadvantage however is that the resulting device is composed of fibers with different numerical aperture creating a mismatch between at least one of the input or output fibers of the coupler and a fiber attached thereto.